Move aws_relational_db_test.py to PkbCommonTestCase
Move aws_relational_db_test.py to PkbCommonTestCase

PiperOrigin-RevId: 400319684

GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker